Electron beam induced deposition (EBID) is a technique used to deposit material on a substrate surface. EBID deposits material on the substrate surface through interaction of the electron beam and a deposition precursor. Often, the precursor is a gas and the material to be deposited is a metal.
Electron beam induced etching (EBIE) is another technique for modification of the surface of a substrate. In EBIE, the electron beam induces etching in the irradiated areas, often assisted by an etching precursor gas.
Electron beam induced deposition is appealing because it enables direct-write nano-fabrication and high resolution visual feedback in a scanning electron microscope (SEM). However, the current applications of EBID are limited.
Applications of EBID have been limited by several problems. Typically, material deposited by EBID suffers from low purity of the intended deposit, causing problems such as high electrical resistance relative to purer deposits. Materials deposited by EBID also tend to have an unfavorable nano/micro-structure, for example, the materials are often polycrystalline. Another limitation of EBID involves the limited number of materials or compounds that can currently be deposited. Deposition of a particular material requires a suitable deposition precursor, which does not exist for some materials. An EBID precursor must react in the presence of the electron beam to form a non-volatile component of the material to be deposited and a volatile portion of material in the compound that is not to be deposited, yet must not react spontaneously with the surface in the absence of the beam. The precursor molecules must adhere to the surface sufficiently to allow the beam to interact, yet not condense on the surface to form a thick layer that obscures the surface.
Material may also be deposited by other techniques, such as chemical vapor deposition (CVD), wet chemistry techniques, and other methods. Such methods are limited by the minimum feature size on which selective deposition can be performed.
One application of EBID is direct write, single step electron beam lithography. Conventional electron beam lithography (EBL) uses an electron irradiation sensitive resist layer which is selectively removed (positive resist), or not removed (negative resist) by a development step following irradiation to define the lithographic pattern. Material is then deposited globally, and the resist removed, leaving deposits only in areas where the resist was removed during development.
The resolution of EBL is limited by the size of the electron beam interaction volume within the resist layer, and the attainable resolution improves with decreasing resist layer thickness. At the limit of very thin resist layers, there are significant practical problems with properly removing the layer to leave well defined, high resolution features after the fabrication step. Multi-step processing with EBL is very difficult due to the multiple resist coating and removal steps involved. Resist layers also tend to obscure alignment marks upon substrates, adding another difficulty to multi-step EBL.
Marbach, “Electron beam induced surface activation: a method for the lithographic fabrication of nanostructures via catalytic processes,” Applied Physics A, Vol. 117, Issue 3, pp. 987-995 (2014), describes a two-step method for deposition on a substrate. Marbach activates a surface without the presence of precursor molecules, followed by deposition of material on the activated areas in a separate step. This method is limited in application because of the limited number of materials that can be grown on areas activated without the use of a precursor gas. In addition, the method of Marbach is not applicable to iterative deposition, needed to fabricate complex, multi-component materials and devices.
Another process for two-step deposition is described by Mackus et al, “Local deposition of high-purity Pt nanostructures by combining electron beam induced deposition and atomic layer deposition,” J. Appl. Physics, Vol. 107, 116102-116102-3 (2010). In Mackus, a substrate is activated by depositing a seed layer of platinum by EBID, followed by selective atomic layer deposition of platinum on the seed layer. While this two-step process provides selective growth of platinum, the process is limited in the materials that can be grown on metallic seed layers. The process is also limited in terms of iterative application of the technique, which is needed to fabricate complex, multi-component structures.
Randolph et al, “Local deposition of high-purity Pt nanostructures by combining electron beam induced deposition and atomic layer deposition,” Particle, Vol. 30, pp. 672-677 (2013), describes a method for deposition of platinum by electron beam fluorination of a surface, followed by a CVD step in which Pt(PF3)4 and XeF2 precursors are mixed to achieve localized deposition of Pt on the fluorinated surface. It is not known whether this process can be used for the deposition of materials other than polycrystalline, porous Pt.
Djenizian et al, “Electron-Beam Induced Nanomasking for Metal Electrodeposition on Semiconductor Surfaces,” J. Electrochem. Soc., Vol. 148, Issue 3, pp. C197-C202, describes a method for creating a “negative resist” by EBID of carbon on a substrate using residual hydrocarbon contamination in an SEM system. The carbon deposited by EBID selectively blocks the electrodeposition of Au in a subsequent step. The method of Djenizian is limited in application for the same reasons as Mackus above.
What is needed are improved techniques for selective nano-deposition of high quality, functional materials with improved resolution, ease of fabrication, fewer processing steps, and the ability to perform multiple processing cycles.